Generally, crawler belts which are used on crawler type vehicles such as hydraulic excavator and the like are largely constituted by rows of endlessly connected right and left track links each formed with an outer link portion and an inner link portions at longitudinally opposite ends, respectively, bushes interposed transversely between the inner link portions of the right and left track links and each having opposite axial ends thereof located face to face with outer link portions of the right and left track links, and connecting pins placed in the respective bushes and each having opposite axial ends thereof coupled with the outer link portions by fitting engagement therewith.
The prior art crawler belt of this sort is lapped around an idle wheel and a drive wheel and rolled on or back between and around these wheels by driving the drive wheel from a hydraulic motor or the like to drive a vehicle body in a forward or reverse direction.
Further, in the case of the prior art crawler belt as mentioned above, annular seal members of substantially M-shape in cross-section are fitted between the outer link portions of the track links and the bushes for the purpose of preventing dirt from getting between the bushes and connecting pins while the vehicle is in travel (e.g., as disclosed in Japanese Utility Model Laid-Open No. S49-113342).
In this case, each seal member is constituted by a first lip portion to be abutted against an inner end face of an outer link portion of track link, a second lip portion to be abutted against an end face of a bush, and an interconnecting bridge portion which is bridged between the first and second lip portions to connect same with each other.
According to another prior art crawler belt seal, an abutting surface (hereinafter referred to as “a seal surface” for brevity) of a lip portion on the side of bush, that is to say, a seal surface to be held in abutting engagement with a bush is provided with a ridge portion at a position which is spaced radially outward from a radially inner end of the lip portion (e.g., Japanese Utility Model Laid-Open No. H1-65773).
By the way, in the case of the prior art crawler belt seal which is described in the above-mentioned Japanese Utility Model Laid-Open No. S49-113342, the wall thickness of the lip portion on the side of bush is partly increased thereby to increase a range of allowance for abrasive wear of the lip portion on the side of bush and to enhance its durability.
However, a study on surface pressures which is exerted on the crawler belt seal of the above described prior art construction reveals that the surface pressure on a lip portion on the side of bush has a tendency to increase in certain localities on the inner peripheral side of the lip portion. Therefore, even if wall thickness of the lip portion on the side of bush is increased as described above, abrasive wear of the lip portion progresses considerably in high surface pressure areas. Therefore, the above-described arrangement does not necessarily help to improve the durability of the crawler belt seal to a sufficient degree.
In addition, the surface pressure on the lip portion on the side of bush is elevated only in limited localities, so that it is difficult to improve the seal capacity or strength to a sufficient degree by the lip portion on the side of bush.
Further, in the case of the crawler belt seal as described in the above-mentioned Japanese Utility Model Laid-Open No. H1-65773, attempts were made to increase the seal capacity of the lip portion on the side of bush by providing a ridge portion on the seal surface at a position which is spaced in a radially outward direction from an inner peripheral end of the seal, for increasing the surface pressure at two different positions, namely, at the ridge portion and at the inner peripheral end portion of the seal.
However, as a hydraulic excavator is steered while it is put in travel on a working site, for example, gap spaces which exist in the thrust direction between outer link portions and bushes of the crawler belt are repeatedly and incessantly subjected to a phenomenon in which each gap space is temporarily opened wide and closed. On such an occasion, the ridge portion on the seal surface is momentarily caused to move away from the bush, allowing dirt to intrude into a space between the ridge portion and the root portion of the seal. Once dirt is allowed to get into a gap space between the ridge and root or inner peripheral end portions of the seal, it is confined there to accelerate abrasive wear of the seal surface.
Besides, the provision of a ridge portion on the lip portion on the side of bush makes the seal complicate in shape as a whole, which would be reflected by lower production efficiency of the seal.